A drawback of lean premixed, low emission combustion systems in gas turbines is that they exhibit an increased risk in generating thermo-acoustically induced combustion oscillations. Such oscillations, which have been a well-known problem since the early days of gas turbine development, are due to the strong coupling between fluctuations of heat release rate and pressure and can cause mechanical and thermal damages and limit the operating regime.
A possibility to suppress such oscillations consists in attaching damping devices, such as quarter wave tubes, Helmholtz dampers or acoustic screens.
A reheat combustion system for a gas turbine with sequential combustion including an acoustic screen is described in the document US 2005/229581 A1. The acoustic screen, which is provided inside the mixing zone and/or the combustion chamber, consists of two perforated walls. The volume between both can be seen as multiple integrated Helmholtz volumes. The backward perforated plate allows an impingement cooling of the plate facing the hot combustion chamber.
To prevent hot gases to enter from the combustion chamber into the damping volume, an impingement cooling mass flow is required, which decreases the damping efficiency. If the impingement mass flow is too small, the hot gases recirculate passing through the adjacent holes of the acoustic screen. This phenomenon is known as hot gas ingestion. In case of hot gas ingestion the temperature rises in the damping volume. This leads to an increase of the speed of sound and finally to a shift of the frequency, for which the damping system has been designed. The frequency shift can lead to a strong decrease in damping efficiency. In addition, as the hot gas recirculates in the damping volume, the cooling efficiency is decreased, which can lead to thermal damage of the damping device. Moreover, using a high cooling mass flow, increases the amount of air, which does not take part in the combustion. This results in a higher firing temperature and thus leads to an increase of the NOx emissions.
A solution to the mentioned issues is described, for example, in the document EP 2295864. Here, a multitude of layers are braced together to form single compact Helmholtz dampers, which are cooled using an internal near-wall cooling technique close to the hot combustion chamber. Therefore, the cooling mass flow can be drastically reduced without facing the problem of hot gas ingestion, leading to less emissions and a higher damping efficiency. As single Helmholtz dampers are used, different frequencies can be addressed separately. Whether single or a cluster of Helmholtz dampers is used, the design is based on an appropriate implementation of a near wall cooling.
Another solution of a high-frequency damping system for a combustor in a gas turbine with a cooled wall part is disclosed in EP 2402658. A plurality of cooling paths extending in axial direction are formed in the combustor wall. The cooling paths are connected to a source of cooling medium, such as steam or cooling air, at the one end and to a cooling medium discharge channel at the other end. The cooling medium flowing through the cooling paths cools the peripheral portions of the through holes to avoid or minimize thermal stress, caused by the hot combustion gases when passing the through holes in case of hot gas ingestion.
The document EP 2362147 describes various solutions on how the near-wall cooling can be realized. The near-wall cooling passages are either straight passages or show coil shaped structures parallel to the laminated plates. A drawback of this solution is that due to the shape of the near wall cooling channels, the component is to be made from several layers, which in the end have to be brazed together. Brazing itself is a well-known technique in the turbo machinery business, but inherits disadvantages while compared to other joining methods.
Another way to realize different shapes of near wall cooling channels in wall structures would be to use a so-called “lost wax casting process”. With this technique, which is widely used to manufacture cooling passages in turbine blades, a ceramic core is used during the casting process to realize the later cooling channels. Compared to casting processes that can avoid the usage of ceramic cores, the production costs are multiple times higher.